The Influence of the Ketogenic Diet on the Immune Tolerant Microenvironment in Glioblastoma

SIMPLE SUMMARY: Preclinical investigations have identified promising anti-tumor activity of a ketogenic diet in glioblastoma. Although a majority of the work has focused on how this diet directly influences tumor growth, our understanding of its impact on the tumor microenvironment remains limited. In this study, although the anti-tumor activity of a ketogenic diet in glioblastoma was validated, we uncovered a paradoxical increase in a “pro-tumoral” immune response, skewing tumor-associated macrophages towards an “M2-like” phenotype. We hypothesized this would limit the anti-tumor activity of a ketogenic diet, which was tested by combining this diet with a CSF-1 inhibitor. The combination of a ketogenic diet with CSF-1 inhibition normalized tumor-associated macrophages and led to a profound improvement in survival in mice. This provides rationale to test this novel combinatorial strategy in glioblastoma and highlights the importance of understanding how a given therapy influences both the tumor and its associated immune microenvironment. ABSTRACT: Glioblastoma (GBM) represents an aggressive and immune-resistant cancer. Preclinical investigations have identified anti-tumor activity of a ketogenic diet (KD) potentially being used to target GBM’s glycolytic phenotype. Since immune cells in the microenvironment have a similar reliance upon nutrients to perform their individual functions, we sought to determine if KD influenced the immune landscape of GBM. Consistent with previous publications, KD improved survival in GBM in an immune-competent murine model. Immunophenotyping of tumors identified KD-influenced macrophage polarization, with a paradoxical 50% increase in immune-suppressive M2-like-macrophages and a decrease in pro-inflammatory M1-like-macrophages. We recapitulated KD in vitro using a modified cell culture based on metabolomic profiling of serum in KD-fed mice, mechanistically linking the observed changes in macrophage polarization to PPARγ-activation. We hypothesized that parallel increases in M2-macrophage polarization tempered the therapeutic benefit of KD in GBM. To test this, we performed investigations combining KD with the CSF-1R inhibitor (BLZ945), which influences macrophage polarization. The combination demonstrated a striking improvement in survival and correlative studies confirmed BLZ945 normalized KD-induced changes in macrophage polarization. Overall, KD demonstrates antitumor activity in GBM; however, its efficacy is attenuated by promoting an immunosuppressive phenotype in macrophages. Combinatorial strategies designed to modulate macrophage polarization represent a rational approach to improve the anti-tumor activity of KD in GBM.